I don't think most people are being judgemental at all, I don't bother to get offended. Maybe because I grew up with everyone naturally assuming the my mom was my grandmother and my brother and sister in law were my parents. Natural assumption as I'm 20 years younger than my brother, 2 years older than his daughter, and actually several years younger than on of my step nephews.
Sometimes assuming someone is a child's parent is based on behaviors -- I'm 12 years older than my best and oldest friend - I babysat her when she was an infant and toddler, and when she had a stroke at age 5, I was a nurses aid at that hospital, and her mother's helper at home. So in public it's obvious that I'm her 'caregiver', but not a payed one if you know what I mean. Her daughter is 14, and yes I am old enough to be grandmother to a 14 year old. So with 14year old calling her mom, and the older lady taking care of her, I'm not going to get bent out of shape if someone asks the teen about her grandmother.